December 23, 1972 ] THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
503 
THE SULPHUROUS IMPURITY IN COAL GAS.* 
UY A. VERNON HARCOURT, ESQ., F.R.S., SEC. CHEM. SOC. 
The luminous flames we use to light our houses after 
sunset are all equally gas flames. 
The gas which burns round the wick of a candle is 
formed by the action of heat on the grease of which 
the candle is made. The gas which burns over the wick 
of an oil lamp is formed by the action of heat on the oil. 
The gas which burns over a “gas-burner” is formed by 
the action of heat on coal. 
By the heating of any of these substances many 
different kinds of gas are produced, some of which 
become liquids if they are allowed to cool, while others 
are permanently gaseous. In the case of candles and 
lamps, when the hot gas is burnt as fast as it is made, 
the uncondensable and the condensable are burnt alike, 
the latter contributing even more than the former to the 
luminosity of the flame. In the case of coal gas, which 
is kept for many hours, and has often to travel along 
many miles of iron piping before it is burnt, only very 
little condensable gas reaches the burner. In this re¬ 
spect “ gas ” illumination is at a great disadvantage as 
compared with candles or lamps. To counterbalance 
this disadvantage gas must be produced from some sub¬ 
stance which is much cheaper than grease or oil, and 
such a substance we have in coal. But coal, like all 
minerals obtained on the large scale, is mixed with small 
quantities of other substances, and, in particular, masses 
of coal always contain a greater or less proportion of a 
mineral known, according to its form, as pyrites or 
marcasite, composed of eight parts of sulphur united 
with seven of iron. When coal is heated, a part of the 
sulphur from this mineral unites with the carbon and 
hydrogen of the coal, and thus the illuminating gas 
formed from coal is contaminated with at least two 
sulphur compounds—carbon bisulphide and hydrogen 
sulphide. Such gas yields when it is burnt, in addition 
to water and carbon dioxide, sulphur dioxide produced 
by the burning of the sulphur. This gas has a well- 
known pungent smell; it acts on various colouring- 
matters, and it is gradually changed in presence of air 
and moisture into a far more destructive substance called 
hydrogen sulphate or oil of vitriol. Unlike sulphur 
dioxide, which is a gas and can be removed by ventila¬ 
tion, hydrogen sulphate is not volatile, and exercises a 
continued corrosive action upon organic materials or 
fabrics on which it is deposited. In a furnished room 
the leather bindings of books and coverings of furniture 
are especially liable to be injured thus; perhaps because 
this material, being a better conductor of heat than 
others which are used for the same purposes, is more 
bedewed after gas has been lighted in a room than they 
are, and also because it requires less often to be cleaned 
or renewed. Fortunately the conversion of sulphur 
dioxide into hydrogen sulphate takes place so slowly 
that in a room lighted with gas and well ventilated a 
very small part of the sulphur which gas is liable to 
contain remains in this destructive form. Fortunately, 
also, the quantity of sulphur which gas as at present 
manufactured is liable to contain is very small. In 
a room lighted with four gas-burners the volume of 
sulphur dioxide mixed with the atmosphere of the room 
in the course of an hour is about one-hundredth of a 
cubic foot. The amount actually present at any moment, 
if the room is fairly ventilated, is a small traction of 
this volume. Nevertheless, it is possible, and even 
probable, that some injury, especially to the bindings of 
books, may in the course of years be caused under these 
circumstances; and it is desirable, with a view to avoid¬ 
ing any such injury, and also with a view to allaying 
the apprehension of it, that the proportion of sulphur 
* Paper read at the Royal Institution, Friday, April 19th, 
1872. 
compounds in coal gas should be reduced as far as. 
possible. 
A ton of coal may contain about 30 lb. of sulphur: it" 
yields nearly 10,000 cubic feet of gas, and a considerable- 
part of the sulphur contained in it is given off with the- 
gas in combination with either carbon or hydrogen. Of 
these two elements, hydrogen claims by far the larger 
proportion, not less than ten parts for one that is united 
with carbon. For the purification of gas from hydrogen, 
sulphide, excellent methods are everywhere in use. The- 
gas is passed through layers of slaked lime or of iron 
oxide, by either of which substances all the hydrogen 
sulphide is capable of being completely absorbed. But 
the smaller quantity of sulphur existing in the form of 
carbon bisulphide is not arrested by these agents, nor is 
there at present any material or process known by which 
it can be effectually removed. 
When coal gas which is pure from hydrogen sulphide 
is heated and tested again, it is found to contain this 
impurity, showing that some ingredients of the gas are 
capable of producing hydrogen sulphide by their mutual 
action. Hydrogen seems to have a much stronger 
affinity for sulphur than carbon has. One consequence 
of this difference is the unequal partition of the sulphur 
between the two elements in the gas retort. But this 
inequality does not reach its limit in the short time which 
elapses between the formation and cooling down of the 
gas ; and accordingly, when foul gas (or gas which has 
not been purified from hydrogen sulphide) is further 
heated, the proportion of hydrogen sulphide in it is in¬ 
creased, and that of carbon tia ulphide diminished. From 
a number of experiments in which foul gas was passed 
through an iron tube three inches in diameter, filled with, 
iron turnings and heated for a length of about two feet, 
to low redness, it appeared that the amount of carbon 
bisulphide could be so far reduced that the gas, after 
purification from hydrogen sulphide, contained five or 
six instead of thirty grains of sulphur in 100 cubifffeet. 
The gas was driven through the heated tube at a rate ofr 
from one to two cubic feet a minute. 
A somewhat greater reduction in the amount of 
sulphur is obtained by heating the gas after, instead of 
before, purification and purifying it a second time. If 
it is the case, as seems probable, that the sulphur present 
in coal gas distributes itself when the gas is heated 
between the carbon and hydrogen in a ratio dependent 
upon the relative affinity for sulphur of the two elements-,, 
the proportion of carbon bisulphide to the total sulphur 
in the gas will be always the same when the composition-, 
of the gas is the same, and when it has been heated hmg 
enough for the establishment of an equilibrium. Accord¬ 
ingly we should expect the removal of sulphur, by the- 
conversion of carbon bisulphide into hydrogen sulphide 
and the absorption of the latter, to be accomplished 
more effectually with gas from which the chief part of 
the sulphur had already been extracted. And this, as. 
has already been stated, is found to be the case. 
The nature of the chemical change which takes place 
when coal gas is heated may be illustrated by passing 
hydrogen over the mouth of the tube containing carbon 
bisulphide, and thence through a piece of combustion 
tubing heated nearly to a red heat. The mixture of 
hydrogen and carbon bisulphide vapour has no action on 
a solution of lead acetate ; but, after the application of 
heat, the gas which issues produces at once a black pre¬ 
cipitate, proving that hydrogen sulphide has been 
formed. This change occurs readily with hydrogen 
which has been carefully dried; but the presence of 
moisture appears to promote it; and as coal gas contains 
a quantity of aqueous vapour, much more than sufficient 
to react with the maximum amount of carbon bisulphide, 
it is possible that the formation of hydrogen sulphide 
when coal gas is heated, may be partly due to the inter¬ 
vention of moisture. 
If clean iron nails are heated to redness in a glass 
tube, and coal gas is passed slowly over them, a soft 
